The present invention relates to a screw press intended for pressing liquid from fibrous slurries, such as paper pulp, sludge obtained from communal and industrial sewage works, suspensions of secondary paper and the like, optionally in conjunction with a washing operation. To this end, the screw press is of the kind in which the material is fed into the press at one end of a press screw arranged within a cylindrical strainer drum and rotatable about its longitudinal axis, and fed out of the press through a discharge zone located at the opposite end of the screw, the core of said screw having a successively increasing diameter along a major part of its length such that the space defined between said core and the wall of said drum gradually decreases in the feed direction, in order to exert a pressing force on the material advanced by the screw.
Simple dewatering and dewatering in combination with pressing are applied, for example, when producing paper pulp, where, during the process, the fibre material is diluted several times with water or caustic liquor, e.g. for screening, bleaching, washing purposes etc., the liquid being pressed out of the material between each such diluting operation. Fibrous sludge obtained from communal and industrial purification works is dewatered and pressed to reduce its weight prior to being transported for destruction or storage.
These processes are, today, effected by means of conventional apparatus, such as drum dewaterers, suction filters, screw-presses, screw dewaterers, disc presses, roller presses and centrifugal devices. Common disadvantages with these apparatus are that they are expensive, large and bulky and that they require a lot of energy for their operation.
Dewatering operations undertaken in combination with washing operations is applied, for example, in the manufacture of semi-chemical and chemical pulp, in which the cooking liquor, subsequent to the digestion stage, contains organic and inorganic substances. These substances must be removed and in the majority of cases are subjected to vaporization and combustion processes in order to recover the heat content of the wood substances dissolved therein and to enable the cooking chemicals to be re-used. To this end, the pulp is separated from the cooking liquor, suitably by means of a washing process. Washing of the pulp from the cooking liquor should be as complete as possible and still be carried out with a minimum dilution. Washing is also necessary from the aspect of environmental care, since excessively high contents of chemicals in the waste water contaminates the water ways, while organic constituents have the drawback that they consume oxygen.
Normally, the aforementioned working operations are, today, effected by means of rotary suction filters having a washing zone, continuous diffusors and washing presses in which the material is compacted in combination with the pressing thereof. These apparatus require an even greater investment than do apparatus for simple dewatering operations. This means, inter alia, that it is not economically justifiable to manufacture such apparatus below a certain size and below a certain minimum capacity. Because of this, the demand is limited in respect of small factories or factories of average size which require apparatus of a smaller size.
A conventional, simple, dewatering screw-press also has the disadvantage that its dewatering capacity is low, primarily at the inlet end of the press where the slurry is of low concentration and thus large quantities of liquid must be drained off. Tests have shown that, inter alia, only a small number of the holes in the draining drum operate effectively, and hence the dewaterer must be given unnecessarily large dimensions in order to obtain a given capacity. Furthermore, the possibilities of regulating the said capacity and the outgoing dryness of the pulp are relatively limited, and hence this type of dewatering apparatus has not been found suitable for dewatering the material in combination with the washing thereof.